The invention concerns a high pressure sodium vapour lamp with an arc tube made from polychrystalline aluminumoxyd (PCA), a fill of sodium and mercury, as well as xenon as a fill gas.
High pressure sodium vapour lamps of this type are known for their high luminous efficiency in the range of the visible spectrum. This is so because the light emission of these lamps takes place in a spectral area which corresponds to the maximum of the graph of eye sensitivity. The most commercially available high pressure sodium vapour-(HPS)-lamps are, therefore, optimized with regard to maximum lamp lumens.
In addition thereto HPS-lamps enjoy a long life and a very small lumen depreciation.
The high luminous efficiency and the long life also render HPS-lamps particularly useful for application in plant growth promotion, despite the fact that their spectrum is optimized for the human eye and not for the favourisation or promotion, respectively, of the plant growth process.
From EP 0 364 014) a method is known to optimize the blue part of the spectrum of a HPS-lamp. It is known that this is important to prevent plants from growing spindle-meagre and with small leaves. The situation in green houses, however, is such that there is a sufficient amount of blue light of the sun present already, even if it is cloudy in wintertime.
The European patent application mentioned describes a HPS-lamp which is optimized with regard to photosynthesis in plants, discloses an optimum sodium-mercury-amalgam-proportion and points out that the PCA arc tube must be made shorter and wider in order to make this lamp electrically compatible with present ballasts. It has been discovered, however, that when trying to follow this way the arc tube becomes short to an extent that increased heat losses at the arc tube ends become apparent and that, hence, the gain of efficiency with regard to photosynthesis gets lost. In addition thereto the low length of the arc tube can result in undesired changes of light distribution in some in the luminaires destined for plant growth. This implies that the known lamp as described is usable in connection with specially devised operating systems only.
The object underlying the invention is seen in provision of a high pressure sodium vapour lamp of the type mentioned above which is optimized with regard to the efficiency of photosynthesis in plants, which is exchangeable with the present lamps, which is compatible with present ballasts, starters and luminaries, and which finally when compared with a conventional HPS-lamp renders a gain as to photosynthetic effect.
This object is met in accordance with the invention by a high pressure sodium vapour lamp having a PCA-arc tube, a fill consisting of sodium, mercury and xenon as a filling gas, with a sodium weight portion within the sodium-mercury-amalgam of approx. 12% to approx. 20% and a xenon filling pressure in the cold state between approx. 180 Torr and approx. 350 Torr, with a D-line reversal width of the radiation spectrum of approx. 130 xc3x85 to approx. 200 xc3x85, and with approx. 14% to approx. 18% radiation portion in the red wave length area 635 nm to 760 nm and with approx. 7% to approx. 10% radiation portion in the blue wave length area 380 nm to 500 nm, each of a radiation power within the total wave length area 380 nm to 760 nm.
To minimize the heat losses towards the walls of the arc tube it has been recognized as effective to fill the tube with a xenon pressure which is as high as possible without endangering the flawless ignition of the lamps by means of the starters which are certified therefore. This leads to an increase of luminous efficiency of approx. 10%. With a lamp in accordance with the invention the xenon pressure should be brought to a value which is as high as possible but at the same time permits the lamps to get perfectly ignited by the starters which are certified for the respective lamp type. In praxi these are superposed pulse igniters usually which show a minimum peak voltage specified in dependency of lamp power.
In addition thereto the discharge length of the lamp according to the invention should not deviate by more than 25% from the discharge length of conventional HPS-lamps of the same power. If one conforms to this, then optical compatibility with present luminaires is secured. The wall loading (lamp power divided by the wall surface of the arc tube between the electrodes) with conventional HPS-lamps is optimized. Notwithstanding that with higher loading the radiated power increases the wall loading must be kept beneath a certain value to secure a long life. This value usually corresponds to a maximum arc tube temperature of 1200xc2x0 and lies between 15 and 25 W/cm2 in dependency of nominal lamp power. With the lamp in accordance with the present invention the wall loading may deviate 10% maximally from the value of the corresponding conventional HPS-lamp. The intended increase of photosynthesis efficiency is then realized by optimizing the composition of the sodium-mercury-amalgam and the D-line reversal width (distance of the tops of the two wings of the sodium-D-line of the radiation spectrum) of approx. 130 xc3x85 to approx. 200 xc3x85. These variables are elected so that photosynthesis efficiency of the lamp is optimized. The photosynthesis efficiency or efficacy ,respectively, is defined as
xcex7ps="PHgr"ps/Pla
wherein
"PHgr"ps=K∫Vps(xcex)Pxcexxcex4xcex is the photosynthetical effective radiation portion expressed in phytolumen, and
Pla is the power distributed within the lamp.
K=1088.4 Phyto-lmxc2x7Wxe2x88x921 
VPS is the spectrally effective function for the photosynthesis in plants indicated in FIG. 1 and Pxcex is the lamp spectrum.
Comparison with lamp efficiency is carried out. Lamp efficiency is defined as
xcex7="PHgr"/Pla,
wherein
"PHgr"Km∫Vx Pxcexxcex4xcex is the luminous flux expressed in lumen and
Km=683 lmxc2x7Wxe2x88x921, and
Vxcexthe degree of spectral sensitivity according to DIN 5031, part 2.